Due to the various scenario of applications, a scalable audio coding system is highly favorable, which is capable of producing a hierarchical bitstream whose bitrates can be dynamically changed during transmission.
For example, MPEG-4 scalable lossless (SLS) coding provides a gradual refinement, from perceptually weighted reconstruction levels provided by the perceptual audio coding (e.g., advanced audio coding, AAC) core bitstream up to the resolution of the original signal. The original signal is transformed by an integer modified discrete cosine transform (IntMDCT), and the resultant IntMDCT spectral data is coded with two complementary layers, including a core MPEG-4 AAC layer which generates an AAC compliant bit-stream at a pre-defined bitrate which constitutes the minimum rate/quality of the lossless bitstream, and a lossless enhanced layer that makes use of bit-plane coding method to produce fine grain scalable to lossless portion of the lossless bitstream.
In the MPEG-4 SLS encoder, the bitrate for different channels of the audio signal is equally distributed for lossy coding. For example, the bitrate assigned to each frame, Br/f, is calculated as
      B          r      /      f        =                    B        r            ×              N                  s          /          f                      S  wherein Br is the total bitrate (kbps), Ns/f is the sample number/frame and S is the sampling rate. If there are two channels, Br/f is evenly distributed to the two channels as
      B    1    =            B      2        =                            B                      r            /            f                          2            .      
For example, if the mid/side joint stereo coding (M/S stereo coding) is utilized, the bitrates assigned to the mid channel and the side channel are identical according to the equation above. The mid channel represents the Average of Left and Right channel data, and the side channel represents the Difference between Left and Right channel data. In another example, the first and the second channels are the left channel and the right channel, and the bitrate is then assigned to the left and right channel according to the above equation.
The lossless bitstream resulting from the SLS encoder can be directly decoded or can be truncated by a truncator. The lossless bitstream is truncated, e.g. for low bitrate applications, wherein the lossless bitstream may be truncated for each frame based on the target bitrate. For a frame, the original lossless bitstream lengths for the first and second channels are represented as BS1 and BS2, respectively. The target bitstream length is denoted as BST. In a standard SLS truncator, the truncated bitrates are allocated as
      BS    1    T    =            BS      2      T        =          min      ⁢              {                              min            ⁡                          (                                                BS                  1                                ,                                  BS                  2                                            )                                ,                                    BS              T                        2                          }            
M/S stereo coding can be used in lossy audio coding as well as lossless audio coding, for example, in MPEG-4 audio scalable lossless coding (SLS). In most cases, there is comparatively little difference between the audio data for the left and right channels; whereas in some other cases, there is much difference between the audio data for the left and right channels. Accordingly, encoding the data into mid and side channels usually results in a situation where the mid channel is much different from the side channel. In this case, evenly distributing bitrates between the mid channel and the side channel in the audio encoding, or evenly distributing truncated bitrates between the mid channel and the side channel, becomes inefficient.